There are many possible causes. Usually the video is "caching" the video stream so it can play uninterupted on your laptop/PC. Actually, "buffering" is more accurate than caching - the point being that the downloaded video needs to stay ahead of the real time video playback. If there are issues with your bandwidth (not enough, latency, jitter, lots of lost packets, etc..), this can lead to pauses on video. The next most common cause would be your processing power, memory, system config and overhead you are running.
Not knowing much about your setup, the first thing I would suggest is that you try to stream a video that you have a known problem playing without a pause from a different location (friends house, wifi hot spot). If it works there, it points to your quality or speed of your internet connection.
Also, if you are having issues with the laptop while doing wireless internet, direct connect with a cable to your modem. In other words, take your router our of the loop to eliminate a failure point. The easiest way to do this is to power cycle your hardware --- turn off the modem, turn off your laptop, wait 2 minutes, turn on the modem, wait 2 minutes (allowing it to fully connect to your internet provider), then turn on your laptop (which is directly connected to the the modem via an ethernet cable, or usb if you have to) and try the your video test again.
I made some assumptions on the above suggestions. Hopefully it will be helpful. Good luck.
Edited by skidding - 9/23/10 at 4:30am